1. Field of the Invention
The present invention relates to a high efficiency DC-DC converter in which a switching loss of a switching element is reduced.
2. Description of the Related Art
A conventional DC-DC converter shown in FIG. 1 includes: a direct current power supply Vin; a first series circuit that is composed of a step-up reactor L1 and a switching element Q0 composed of a MOSFET or the like and is connected to both ends of the direct current power supply Vin; a rectifying/smoothing circuit that is composed of a rectifier (diode) D1 and a smoothing capacitor Co and is connected in parallel to the switching element Q0; an output voltage dividing series circuit that is composed of a first resistor R1 and a second resistor R2 and is connected in parallel to the smoothing capacitor Co; and a control circuit 10a that controls ON/OFF of the switching element Q0 based on a divided voltage of the first resistor R1 and the second resistor R2.
A description will be made of operations of the conventional DC-DC converter of FIG. 1 while referring to operation waveforms of the respective portions, which are shown in FIG. 2. In FIG. 2, Vo indicates a both-end voltage of the smoothing capacitor Co, IL1 indicates a current flowing through the step-up reactor L1, ID1 indicates a current flowing through the rectifier D1, ID indicates a drain current of the switching element Q0, VDS indicates a drain-source voltage of the switching element Q0, and Vg indicates a gate drive signal of the switching element Q0.
First, in a period TM1, when the switching element Q0 turns on, the currents IL1 and ID flow through a channel of Vin (positive electrode)→L1→Q0→Vin (negative electrode).
Next, in a period TM2, when the switching element Q0 turns off, the currents IL1 and ID1 flow through a channel of Vin (positive electrode)→L1→D1→Co→Vin (negative electrode). Therefore, the drain-source voltage VDS of the switching element Q0 rises.
In the ON period TM1 of the switching element Q0, the current IL1 flowing through the step-up reactor L1 changes with time at an inclination of Vin/L1. Specifically, the current IL1 flowing through the step-up reactor L1 is the minimum immediately after the switching element Q0 turns on, and is the maximum immediately before the switching element Q0 turns off. Therefore, in the conventional DC-DC converter, a loss generated when the switching element Q0 turns off is larger than losses when the switching element Q0 turns on and remains on. In order to reduce the large loss caused when the switching element Q0 turns off, an inductance value of the step-up reactor L1 just needs to be set large, whereby the current flowing through the step-up reactor L1 when the switching element Q0 turns off just needs to be reduced.
However, when the inductance value of the step-up reactor L1 is set large, increases of an iron loss and copper loss of the step-up reactor L1 are brought about. Hence, in order not to allow the increase of either the iron loss or the copper loss, it is necessary to change the step-up reactor L1 to a larger-size one. Moreover, as a method of reducing the loss caused when the switching element Q0 turns off, it is also considered to raise a switching frequency of the switching element Q0. However, when the number of switching times increases, a switching loss also increases. Therefore, there is such a problem that heat generation of the switching element Q0 increases.
Moreover, when the inductance value of the step-up reactor L1 is set large or the switching frequency is set high, the current flowing through the step-up reactor L1 causes direct current superposition.
Then, the switching loss caused when the switching element Q0 turns on increases, and in addition, a surge current caused by a recovery current of the rectifier D1 flows through the switching element Q0. Therefore, large electric noise is generated.
Note that a DC-DC converter is known, which operates in synchronization with timing when the current flowing through the step-up reactor L1 becomes zero for the purpose of suppressing the surge current by the rectifier D1 (refer to Japanese Patent Application Laid-Open Publication No. 2000-78836 (Patent Publication 1)).